Optical scanning devices are well-known in the art and produce machine-readable data signals representative of an object or document being scanned by projecting an image of the document onto a photosensitive detector. The electrical signals produced by the photosensitive detector may then be digitized and processed as necessary to produce an image of the scanned object on a suitable display device, such as, for example, the display of a personal computer. If the object being scanned is text, then the data signals may be converted into text data by a suitable optical character recognition (OCR) program or device.
A typical optical scanner may include illumination and optical systems to accomplish scanning of the object. The illumination system illuminates a portion of the object (referred to herein as a “scan region”), and the optical system collects light (referred to herein as “image light”) reflected by the illuminated scan region and focuses an line across the entire length of the object (referred to herein as a “scan line”) onto the surface of the photosensitive detector. The illumination system generally provides a xenon lamp, a fluorescent lamp, and/or the like to provide a light source. The light source generally provides a light signal that is relatively constant across its surface area. Also, known optical scanners utilize a scan line having a length corresponding to the maximum expected document width, typically about 8.5 inches.
Known scanners capture image light utilizing a charge-coupled device (CCD). A charge-coupled device is a semiconductor device that is typically much smaller than the scan line. Accordingly, the optical system generally comprises an optical component (such as a lens) to perform image reduction. Specifically, the optical component causes the image light associated with the full width of the scan line to be optically reduced so that the received image light is captured by the smaller surface of the CCD semiconductor device.
Also, known scanners implement the scanning CCD as a single series of elements or pixels. Individual pixels or elements of the CCD are in essence photosensitive capacitors. Specifically, the pixels proportionally convert incident light received on a specific segment of the scan line into an electronic charge by trapping the charge in a depletion region of semiconductor material. Since each element or pixel acts in much the same way as a capacitor, a CCD requires an amount of time to receive sufficient light to capture an image. Eventually, the amount of incident light received over a period of time by a CCD is determined by sampling voltages associated with each pixel or element. The sampled voltages represent the brightness of the light received at the respective segments of the CCD.
Image data representative of the entire object may be obtained by sweeping the scan line across the entire object, usually by moving the illumination and optical systems with respect to the object, although it is possible to move the object instead. The sweeping process includes moving the scan line to a specific location, illuminating the locations, allowing the CCD to charge, serially sampling and outputting the voltages. The sampled voltages are converted to digital data via an A/D converter. The digital data is outputted for storage in memory.
However, the geometry associated with known scanning applications is problematic. Specifically, the use of a relatively small CCD element with an associated reduction optical component creates a “roll-off” effect. Due to the geometry of the system and the optical properties of the reduction components, significantly less light is captured from portions of the scan line disposed at more extreme angles. Specifically, significantly less light is captured at the edge of the image than at the center of the image.
Known optical scanning applications attempt to compensate for the roll-off effect by digital signal processing. The digital data is modified by increasing the values associated with specific pixels by a function of the pixel distance from the center of the CCD. However, this adaptation is not an optimal solution. Specifically, the digital signal processing does nothing to augment the reduced signal-to-noise ratio associated with the roll-off effect. Secondly, the digital compensation approach places greater computational complexity into the scanning process.
Alternatively, known optical scanning applications insert attenuating apertures in the light path to adjust to the roll-off effect. A lens associated with the optical system is shaped so as to limit the amount of light received from the middle of the lens. The variation of the lens aperture creates a masking effect, i.e. a portion of light received from the center of the scan line is not received by the CCD. This does create greater uniformity of received image light. However, this occurs at the expense of wasting power associated with the image light, thereby decreasing the signal-to-noise ratio and image quality.